My contract has not been cancelled, despite my request to do so at the end of April!

I contacted Tracker late in April because I was selling my car and needed to inform them that i would no longer need the tracker. I was informed that i needed to pay an early exit fee of R1200. I asked the agent to send me that in writing, which she said she could not do, but her supervisor would call me. I never received a call from the supervisor. I received several calls from Tracker regarding the fitting of a Tracker in my new vehicle, which I eventually declined due to a tracking device being part of the new insurance package. I also informed them that i was still waiting for someone to contact me to cancel the current contract. Nobody ever did. The vehicle was sold at the end of April. They didn't do their job, yet they still expect me to pay for the service that i told them i need to terminate! They have still been debiting my account. I have put a stop payment on the debit order. I have been getting alerts regarding my vehicle. On one occasion i spoke to an operator, and explained the story - this was last week. So I finally got a call from another agent, who sent me the banking details where i could deposit the R 1200, but no letter stating full and final settlement. When i responded asking for this, the amount went up to R1627.58! I am certainly not paying extra because basically Tracker is trying to get out as much money as they can from me before they finally cancel - I asked for this to be done by the end of April. Can somebody at Tracker actually do their job properly and put an end to this sorry saga?